In a typical method for producing multilayer ceramic capacitors, a plurality of dielectric ceramic green sheets of barium titanate ceramics or the like are prepared, and a conductive paste for internal electrode is printed on each of the sheets so as to have a predetermined pattern. Then, the sheets are stacked and pressed to prepare a laminated product wherein the dielectric ceramic green sheets and the layers of the conductive paste are alternately laminated. The laminated product is cut into a plurality of chips having a predetermined shape. The chips are simultaneously burned at a high temperature to prepare the element assemblies of multilayer ceramic capacitors. Then, a conductive paste for external electrode, which contains a conductive powder, a glass powder and an organic vehicle as principal components, is applied on the end face of each of the element assemblies, which allows the internal electrode to expose, to be dried, and then, burned at a high temperature to prepare an external electrode. Thereafter, a coating of nickel, tin or the like is formed on the external electrode by electroplating or the like if necessary.
As conventional metallic materials for use in such conductive pastes for forming internal electrodes, such as multilayer ceramic capacitors, there have been used palladium, silver-palladium, platinum and so forth. However, these metals are expensive noble metals, so that there is a problem in that the production costs of the conductive pastes are increased. For that reason, in recent years, base metals, such as nickel and copper, are mainly used. At present, there are mainly used fine particles of nickel (fine particles of nickel generally having a mean particle diameter of 0.1 to 0.5 micrometers in accordance with the size and capacity of multilayer ceramic capacitors). Since copper has a higher electrical conductivity and a lower melting point than those of nickel, it can improve the characteristics of multilayer ceramic capacitors, and it can contribute to energy saving for production, such as the reduction of burning temperature, so that it is expected as one of favorable metallic materials for internal electrodes in future.
On the other hand, in recent years, in order to increase the capacity of multilayer ceramic capacitors or the like and to decrease the size thereof, it is desired to decrease the thickness of internal electrodes. It is also desired to provide multilayer ceramic capacitors and so forth which have a low internal inductor and which have can be used up to GHz order as high-frequency characteristics, since the field of application of multilayer ceramic capacitors and so forth is enlarged.
In such background, it is desired to provide fine particles of copper having such characteristics that they are monodisperse fine particles having a sharp particle size distribution and containing no coarse particles.
At present, fine particles of copper are mainly used for conductive pastes for external electrodes of multilayer ceramic capacitors and so forth. Such fine particles of copper have particle diameters of about 0.5 to 10 micrometers in accordance with the size of multilayer ceramic capacitors and so forth, and have various shapes, such as spherical, flaky and undefined shapes. The fine particles of copper having the above-described size and shape are mixed in typical conductive paste for external electrodes to be used.
As methods for producing such fine particles of copper, there are proposed a method for reducing a copper sulfate solution with L-ascorbic acid or L-ascorbate (see, e.g., Japanese Patent Laid-Open No. 63-186803), a method for reducing a copper sulfate solution with D-erythorbic acid or D-erythorbate (see, e.g., Japanese Patent laid-Open No. 63-186805), a method for reducing a copper sulfate solution with a borohydride compound (see, e.g., Japanese Patent Laid-Open No. 63-186811), a method for reducing a copper sulfate solution with an aromatic compound having a hydroxyl (—OH) group (see, e.g., Japanese Patent Laid-Open No. 1-225705), a method for adding a reaction initiator to an aqueous mixed solution containing copper ions, a reducing agent and a complexing agent to allow reduction, and then, for adding copper ions, a reducing agent and a pH adjustor to the solution to produce a fine powder of copper (see, e.g., Japanese Patent Laid-Open No. 63-274706), a method for mixing an alkali hydroxide to an aqueous copper salt solution containing cupric ions to generate cupric oxide, adding a reducing sugar to the solution to reduce cupric oxide to cuprous oxide, and then, adding a reducing agent of hydrazine to the solution to reduce cuprous oxide (see, e.g., Japanese Patent Laid-Open No. 2003-342621), a method for allowing copper oxide to react with a reducing agent, such as hydrazine, in a solvent containing a sulfur compound and a protective colloid to produce fine particles of copper (see, e.g., Japanese Patent Laid-Open No. 2004-256857) and so forth.
However, the fine particles of copper obtained by the method disclosed in Japanese Patent Laid-Open No. 63-186803 have a mean particle diameter of 1.0 to 1.8 micrometers, and are not sufficient to be used as fine particles of copper for internal electrodes. In addition, since the pH-adjusted aqueous copper ion solution and the pH-adjusted aqueous reducing agent solution are used for reducing copper ions to copper particles via cuprous oxide in this method, the control of particle diameters is unstable, and aggregation (bonding of particles) is caused so as not to obtain a constant shape, so that there are some cases where the particle size distribution is broader.
The fine particles of copper obtained by the method disclosed in Japanese Patent Laid-Open No. 63-186805 have a mean particle diameter of 0.8 to 2.0 micrometers, and are not sufficient to be used as fine particles of copper for internal electrodes. In addition, since the pH-adjusted aqueous copper ion solution and the pH-adjusted aqueous reducing agent solution are used for reducing copper ions to copper particles via cuprous oxide in this method, the control of particle diameters is unstable, and aggregation (bonding of particles) is caused so as not to obtain a constant shape, so that there are some cases where the particle size distribution is broader.
The fine particles of copper obtained by the method disclosed in Japanese Patent Laid-Open No. 63-186811 have a mean particle diameter of 0.3 to 0.7 micrometers, which is smaller than that of the fine particles of copper obtained by each of the methods disclosed in Japanese Patent Laid-Open Nos. 63-186803 and 63-186805, but they are not sufficient to be used as fine particles of copper for internal electrodes. In addition, since the borohydride compound is used as a reducing agent, there are some cases where workability and stability are deteriorated by causing autolysis if the pH of the reducing agent is low when the pH is adjusted. On the other hand, if the pH of the reducing agent is raised, the borohydride compound is stabilized. However, since the reduction of copper ions is carried out via cuprous oxide in this case, the control of particle diameters is unstable, and aggregation (bonding of particles) is caused so as not to obtain a constant shape, so that there are some cases where the particle size distribution is broader.
The fine particles of copper obtained by the method disclosed in Japanese Patent Laid-Open No. 1-225705 have a mean particle diameter of 0.7 to 1.5 micrometers, and are not sufficient to be used as fine particles of copper for internal electrodes. In addition, hydroquinone is used as a reducing agent, so that it is difficult to further decrease the size of the fine particles of copper even if the pH and temperature in reaction are controlled. Moreover, since the pH-adjusted aqueous copper ion solution and the pH-adjusted aqueous reducing agent solution are used for reducing copper ions to copper particles via cuprous oxide in this method, the control of particle diameters is unstable, and aggregation (bonding of particles) is caused so as not to obtain a constant shape, so that there are some cases where the particle size distribution is broader.
The fine particles of copper obtained by the method disclosed in Japanese Patent Laid-Open No. 63-274706 have a mean particle diameter of 0.16 to 0.61 micrometers. It is considered that they can be used as a copper powder for internal electrodes judging from the mean particle diameter. However, in this method, the reduction of copper ions is carried out in a high pH range (pH 12-13.5), so that copper ions are reduced to copper particles via copper hydroxide, copper oxide and cuprous oxide. For that reason, the control of particle diameters is unstable, and aggregation (bonding of particles) is caused so as not to obtain a constant shape, so that there are some cases where the particle size distribution is broader.
The fine particles of copper obtained by the method disclosed in Japanese Patent Laid-Open No. 2003-342621 have a mean particle diameter of 0.5 to 4.0 micrometers, and are not sufficient to be used as fine particles of copper for internal electrodes. In addition, in this method, cuprous oxide generated from bivalent copper ions are reduced to cupric oxide, and thereafter, cupric oxide thus obtained are further reduced to copper particles. The reduction of cupric oxide to copper particles is a so-called dissolving and depositing reaction. If this method is used for producing copper particles having a large particle diameter to some extent, the control of particle diameters can be stably carried out, and the particle size distribution can be sharp. However, in this method, it is difficult to obtain fine particles of copper for internal electrodes, and it is difficult to obtain fine particles which are separated from each other (the fine particles containing no intergrowth particles and no aggregation particles).
The fine particles of copper obtained by the method disclosed in Japanese Patent Laid-Open No. 2004-256857 have, as mean particle diameters, a primary particle diameter of 0.25 to 0.5 micrometers and a secondary particle diameter of 0.3 to 0.6 micrometers. It is considered that they can be used as a copper powder for internal electrodes judging from the mean particle diameters. In addition, the tap density of the copper powder is in the range of from 3.2 g/cm3 to 3.4 g/cm3 which is a high tap density as fine particles, so that it is considered that the dispersibility of the copper powder is excellent. However, since the reaction in the method disclosed in Japanese Patent Laid-Open No. 2004-256857 is allowed out in the presence of the sulfur compound, there is some possibility that the internal and surface of the fine particles of copper may contain the sulfur compound. Since sulfur generally has a bad influence on the reliability of electronic parts, it is not desired that a copper powder for conductive paste contains sulfur.